Make signals include the object they are attached to (pool, report)

This means pushing the types into the common types module, which
required a refactor of a bunch of other libraries.

db/migrations/00123_signal_drop_title.sql

@@ -0,0 +1,4 @@
+-- +goose up
+ALTER TABLE signal DROP COLUMN title;
+-- +goose down
+ALTER TABLE signal ADD COLUMN title TEXT NOT NULL;